Conventionally, screw compressors have been used as compressors for compressing refrigerant or air. For example, Japanese Patent Publication No. 2004-316586 describes a single screw compressor including one screw rotor and two gate rotors.
The single screw compressor will be described hereinafter. The screw rotor is generally cylindrical in shape, and has a plurality of helical grooves foamed in its outer circumferential surface. The gate rotors are formed in a generally plate-like shape, and are lateral to the screw rotor. The gate rotors each include a plurality of radially arranged gates each having the form of a rectangular plate. The gate rotors are placed in a position where their rotation axes are orthogonal to the rotation axis of the screw rotor. The gates are meshed with the helical grooves of the screw rotor.
The single screw compressor includes the screw rotor and the gate rotors all contained in a casing. The helical grooves of the screw rotor, the gates of the gate rotors, and the inner wall of the casing define a compression chamber. When the screw rotor is rotationally driven by, e.g., an electric motor, the gate rotors rotate with the rotation of the screw rotor. Thus, the gates of the gate rotors relatively move from the beginnings of the helical grooves with which the gates mesh (the end of the suction side of the compressor) to the ends thereof (the end of the discharge side of the compressor). This allows the volume of the completely-closed compression chamber to gradually decrease. As a result, fluid in the compression chamber is compressed.
As described in Japanese Patent Publication Nos. 2004-316586 and 2005-030361, screw compressors include capacity control slide valves. Each of the slide valves is positioned to face the outer circumference of a screw rotor, and is slidable along the rotation axis of the screw rotor. Meanwhile, the screw compressors include bypass paths each formed to provide communication between a compression chamber during a compression phase and the suction side of the compressor. Movement of the slide valve changes the area of the opening of the bypass path in the inner circumferential surface of a cylinder into which the screw rotor is inserted. Thus, the volume of fluid which is returned through the bypass path to the suction side changes. As a result, the volume of compressed fluid which is finally discharged from the compression chamber changes, and thus, the volume of fluid discharged from the screw compressor (i.e., the capacity of the screw compressor) changes.